Aerosol effects on cloud dynamics, microphysics and precipitation: numerical simulations with WRF with spectral (bin) microphysics

Aerosol effects on cloud dynamics, microphysics and precipitation are investigated using spectral (bin) microphysics (SBM) coupled with the Weather Research Forecast Model (WRF). The SBM is based on solving an equation system for size distribution functions of water drops, three types of ice crystals, aggregates, graupel and hail/frozen drops. Special attention is paid to the description of cloud-aerosol interaction. Atmospheric aerosols are described by a special size distribution function. All size distributions are approximated on mass grids containing 33 bins. Simulations are performed for several typical cloud types and cloud systems: orographic clouds, supercells (thunderstorms) and maritime convective clouds, using two- and three-dimensional versions of WRF. Two different aerosol conditions were specified: maritime or continental. Initially, continental aerosols lead to a delay in the formation of precipitating clouds. This allows liquid drops to reach higher levels and to grow by diffusional growth, which can invigorate convection, leading to greater production of ice hydrometeor mass. In squall lines, for example, production of ice mass can come at the expense of liquid precipitation at the ground, especially when convection forms with large wind shear. However, over longer time scales, e.g, with cloud formation over mountains in a nearly constant background wind, it is shown that aerosols tend to invigorate cloud formation downwind on the mountain slope, leading eventually to greater rain production at the surface. The formation of precipitation can be further delayed in dry air, and this occurs more so in continental versus maritime aerosols. Since relative humidity depends of the sensible fluxes from the surface, precipitation amount and distribution turns out to be dramatically dependent of the surface heating by solar radiation. The main conclusion is that aerosol effects on precipitation dramatically depend on environmental conditions and thermodynamic factors. Accumulated (monthly, annual, etc.) aerosol effects on precipitation depend on typical (climatic) conditions of a particular geographic region. Results obtained using SBM are compared to those obtained using bulk-parameterization schemes. Possible reasons for the difference in the results are discussed.